The present invention relates to a rotary feed-through for mounting a rotating substrate tube in a lathe and providing a flow of process gas into the substrate tube. The present invention further relates to a method for manufacturing a preform for optical fibres using Chemical Vapour Deposition, CVD, and a lathe comprising the new rotary feed-through.
The deposition of glass layers on the interior of a substrate tube, wherein one or more reactive gases and an oxygen-containing gas are supplied to said substrate tube, is known per se, for example from U.S. Pat. No. 6,260,510 in the name of the present applicant. According to the method that is known therefrom, layers of silicon dioxide, which may or may not be doped (e.g. germanium-doped silicon dioxide), are coated onto the interior surface of a substrate tube consisting of quartz glass, for example. Such a deposition reaction may be carried out by positioning the substrate tube along the cylindrical axis of the resonant cavity and subsequently flushing the inside of the tube with a gaseous mixture comprising oxygen, silicon chloride and germanium chloride, for example. Following that, a localized plasma is generated within the substrate tube so as to produce direct deposition of germanium-doped silicon dioxide on the interior surface of the substrate tube. Since such deposition only occurs in the vicinity of the localized plasma, the resonant cavity (and thus the plasma) must be reciprocated along the cylindrical axis of the substrate tube in order to coat the substrate tube uniformly along the entire length thereof. When the deposition of the layers is completed, the substrate tube is thermally treated in such a manner that it will contract into a rod, which rod is also called an optical preform. If the end of the optical preform is heated in such a manner that said end starts to melt, an optical fibre can be drawn from the rod and be wound onto a reel. Such an optical fibre thus has a core-cladding portion corresponding to that of the optical preform. Because a germanium-doped core has a higher refractive index than the undoped cladding, for example, the fibre can act as a waveguide, viz. for use in propagating optical telecommunication signals. It should be noted, however, that the gaseous mixture that is flushed through the inner part of the substrate tube may also contain other components; a fluorine-containing compound may be added, causing a reduction in the refractive index of the doped silicon dioxide.
The use of such a fibre for telecommunication purposes requires the fibre to be substantially free from contamination, since such contamination can cause serious attenuation of the signal being carried if great fibre lengths are used. As a result, it is important not only that the aforesaid PCVD process be highly uniform, but also that the reactive gases used for the deposition do not contain any undesirable impurities. During the aforesaid chemical vapour deposition, the hydrogen atoms can thus form —OH-bonds in the glass layers that have been deposited on the interior of the substrate tube, which —OH-bonds have a strongly adverse effect on the transmission spectre of a fibre drawn from an optical preform, in particular on account of the strong absorption thereof at 1250 nm and 1385 nm. Such absorption losses due to the presence of small amounts of impurities in the gaseous starting material can amount to 10-20 dB/km of a wavelength of 1385 nm. Although prior art methods exist for preventing the incorporation of such —OH-groups into the optical glass fibre, for example by carrying out a chlorination step following the deposition step in the case of porous glass structures, as known from U.S. Pat. No. 4,675,038, or by adding fluorine during the chemical vapour deposition reaction, for example, as known from European patent application No. 0 127 227, both prior art methods have this drawback that an additional amount of chlorine or fluorine, respectively, will find its way in the final glass structure, leading to increased attenuation losses caused by Rayleigh scattering.
Light guidance takes place in a small part of an optical glass fibre, viz. the optical core, and a small part of the cladding surrounding said core. It is important, therefore, that optical preforms from which an optical glass fibre is drawn, which glass fibre is responsible for the light conduction, be free from impurities, in particular hydroxyl groups.
European patent application No. 0 401 742 relates to an OVD process wherein silicon dioxide free from hydroxyl ions is deposited on a substrate, which substrate is localized in a space that is separated from the surrounding atmosphere.
European patent application no. 0 477 4996.5, in the name of the present applicant discloses a method to combat the above mentioned contaminations, wherein a method is disclosed for manufacturing a preform for optical fibres, wherein deposition of glass-forming compounds on a substrate tube takes place, wherein the deposition step is carried out in an environment in which the substrate is present in a conditioned atmosphere, which conditioned atmosphere has a moisture content lower than that of the non-conditioned atmosphere.
The downside of such a method is that the complete CVD lathe thus has to be placed in a conditioned atmosphere, such that no contamination is able to enter the substrate tube. This may be a cumbersome and radical process as this would probably involve a redesign of the factory layout.
One aspect of the present invention is thus to provide for a rotary feed-through for mounting a rotating substrate tube in a lathe and providing a flow of process gas into the tube, wherein no contamination is able to enter the substrate tube via the rotary-feed through.
Another aspect of the present invention is to provide for a Chemical Vapour Deposition, CVD, lathe comprising such a rotary feed-through.
Yet another aspect of the invention is to provide for a method for manufacturing a preform for optical fibres using the new Chemical Vapour Deposition, CVD, lathe, wherein deposition of glass-forming compounds on a received substrate tube takes place.
The above mentioned objects are achieved by the present invention.